The protein concentration and colloidal osmotic pressure of avian plasma is about one-half that of mammals. The systemic arterial pressures of birds are, however, considerably higher. It is surprising to find this difference in light of Starling's hypothesis on the equilibrium between colloidal osmotic pressure and hydrostatic pressure within blood capillaries. The objective of this proposed research is to elucidate the transcapillary fluid exchange in adult birds and its development in embryonic birds by determining the colloidal osmotic pressure of plasma proteins, capillary blood pressures, interstitial pressure and capillary permeability. It should resolve the question of a possible imbalance of pressures at the capillary membranes by comparing colloidal osmotic pressure with values for the capillary blood pressure. The latter is determined by either a micro-puncture or an indirect micro-occlusion technique. Capillary permeability, also measured with a micro-occlusion method, can be compared with values for mammals. The use of chicken embryos may result in an ideal system to test the activities of various sympatho- and parasympathomimetic agents and drugs which affect capillary permeability.